The present invention relates to switching power converters. More particularly, the present invention relates to limiting current inrush upon start-up of a switching power converter.
In a typical switching power converter, a duty cycle of a main power switch is controlled in response to a level of an output voltage developed by the power converter in order to regulate the output voltage. When power converter starts up, the output voltage is initially nearly zero. Accordingly, a large voltage may be applied across elements of the converter, resulting in a large inrush current. As a result, the current level may cause a circuit breaker that supplies power to the converter to trip or may cause damage to the converter itself.
A conventional technique for limiting the inrush current includes positioning a current-limiting resistor to receive the input current upon start-up and then shorting the resistor with a switch. Another technique includes placing a current-limiting resistor in parallel with a current-sensing resistor that is positioned to receive the input current. Upon start up, a switch in series with the sensing resistor directs the input current through the larger, current-limiting resistance. Then, the switch directs the input current through the sensing resistor.
A drawback to these techniques is that a dedicated pin would be required to implement them using an integrated circuit controller for the power converter.
In addition, in a conventional switching power converter, control circuitry, such as integrated circuit controller may obtain its operating power from an output of the power converter. During start-up, however, the output is not yet available. Thus, a bleed resistor is sometimes provided to obtain power from an input power source for the power converter, such as the AC supply.
A drawback to this technique is that during normal operation, the bleed resistor continues to draw power from the source, wasting energy. This is especially disadvantageous for devices for which a low energy consumption rating is desired.
An alternate technique involves shorting the bleed the resistor with a switch under normal operating conditions. However, this requires a dedicated pin to control the switch when the using an integrated circuit controller for the power converter.
Therefore, what is needed are improved techniques for preventing inrush current and for preventing power dissipation in a bleed resistor that do not suffer from the aforementioned drawbacks. It is to these ends that the present invention is directed.
The present invention is a current inrush limiting technique for a switching power converter. In one aspect of the invention, a switching power converter includes a main power switch and a current sensor, such as a sensing resistor, that forms a signal representative of an input current. When the input current exceeds a first threshold, the main power switch is opened. When the input current exceeds a second threshold, higher than the first threshold, a current-limiting resistance is coupled to receive the input current. Accordingly, the input current is limited in two stages by two different techniques.
In accordance with another aspect of the invention, a current sensor determines when a power converter is in a start-up mode. In response, a current-limiting resistor is coupled to receive the input current. In addition, a bleed resistor receives current from a power source for providing power to a controller for the power converter. After start-up, such as when an output voltage of the power converter is available to provide power to the controller, the current-limiting resistor is shorted and the bleed resistor is effectively removed. A single pin of an integrated circuit controller controls shorting of the current-limiting resistor and removal of the bleed resistor.